It is known in principle from U.S. Pat. No. 4,761,545 (Marshall, Ricca and Wang) that several ion types can be simultaneously excited by a frequency mixture and, if so wanted, eliminated from the storage cell. The patent explicitly refers to both ion cyclotron resonance spectrometers (ICR) and quadrupole ion traps. From EO 0 362 432 A1 (Franzen and Gabling), it is known that undesired ions can be eliminated by this means during storage in ion traps, that their secular oscillation frequency is excited by dipolar additional alternating fields in such a manner that they enlarge the amplitude of their secular oscillation by resonant energy absorption in the axial alternating field and thus leave the storage field. Quadrupolar excitation with this aim in view is also known from U.S. Pat. No. 5,134,286 (Kelley).
A method of this kind can be used to eliminate the ions of an individual mass, for instance, by applying a single frequency to the two end caps of an ion trap, which is in resonance with the secular oscillation .omega..sub.z of the undesired ion in the z-direction of the ion trap (direction of the axis through the two end caps). The ions of several masses can, however, also be simultaneously eliminated by applying several frequencies at the same time. In extreme cases, all ions except those of one or more desired masses can be prevented from storage by frequency mixtures with one or more gaps so that only the ions of the desired masses are stored. The necessary frequency mixture of the additional field can be calculated (for example, as stated in U.S. Pat. No. 4,761,545) or determined in experiments. In U.S. Pat. No. 5,134,286 (Kelley) white noise is used, in which frequency gaps ("notches") are generated by filtering. The notches then determine the ion species remaining in the ion trap.
For storage the ions can be generated in known manner both internally in the ion trap itself and externally in a special ion source. For internal generation the substances to be ionized are introduced into the ion trap in vapor form. Electrons can then be shot into the ion trap through openings, where they ionize the substance molecules in the usual manner. Other known methods of internal ionization can, however, also be used such as chemical ionization (CI) by reactant gas ions or photoionization by high-energy photons, for example from a LASER. Different kinds of ion generation from surfaces can also be carried out in the ion trap.
Any ion sources can be used for external ionization. The ions are focused into the ion trap after their generation with low kinetic energy, slowed down in it by a collision gas, and thus trapped.